This application relates generally to turbocharged internal combustion engine systems. The application relates more particularly to such engine systems that employ a catalytic treatment device in the exhaust system.
Turbochargers are employed with some internal combustion engines in order to boost the performance of the engine. Exhaust gases discharged from the cylinders of the engine are collected in an exhaust manifold and are supplied to the turbine of the turbocharger to drive the turbine wheel, which in turn drives the compressor wheel of the turbocharger. The compressor pressurizes air and supplies it to the intake manifold of the engine. The overall pressure ratio across the internal combustion engine is thereby increased, allowing the engine to develop increased torque and power output.
As a result of governmental regulations aimed at reducing harmful exhaust emissions from internal combustion engines, internal combustion engine systems nearly always include a catalytic treatment device (commonly referred to as a catalytic converter) in the exhaust system. Within the catalytic treatment device, the exhaust gases flow through a dense honeycomb structure made from a ceramic that is coated with catalysts such as platinum and palladium. Typically, there are two or more different catalysts, such as one that catalyzes a reduction process to convert nitrogen oxides into nitrogen and oxygen, and another that catalyzes an oxidation process to convert carbon monoxide into carbon dioxide.
There is a minimum temperature (called the “light-off temperature”) that the catalysts must be heated to before they become effective to convert the NOx and CO in the exhaust gases into harmless substances. Consequently, during a “cold start” of the engine (for example, after the vehicle has been parked overnight and is started in the morning), there is a period of time before the catalysts have reached their light-off temperature, during which the catalytic treatment device is not effective. This is a significant problem because it has been estimated that as much as 80 percent of total vehicle engine emissions are caused by engine operation during the cold-start period when the catalytic device is ineffective. It is desired to minimize this time period as much as possible.
The present application describes embodiments of turbocharged internal combustion engine systems that employ advantageous means for achieving rapid light-off of a catalytic treatment device.